1. Field of the Invention
The present invention relates generally to an image sensing apparatus, and more particularly, to an image sensing apparatus such as video camera having an automatic focusing function of automatically matching the focus relative to an object in response to a video signal obtained from an image sensor.
2. Description of the Background Art
Conventionally, in an automatic focusing apparatus used in an image sensing apparatus such as video camera, an approach utilizing a video signal itself obtained from an image sensor for evaluating a state in which the focus is controlled has been developed. According to such an approach, a lot of good characteristics can be obtained. For example, there exists no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be precisely matched. Furthermore, according to this approach, a specific sensor for automatic focusing need not be separately provided, so that the apparatus is very simple as a mechanism.
As such a focus control method utilizing a video signal, a control method referred to as a so-called hill climbing servo system has been conventionally known, which is disclosed in, for example, Japanese Patent Laying-Open No. 105978/1986. Briefly stated, in such a conventional method, a level of a high-frequency component of a video signal obtained from an image sensor within the range of a focusing area set in the center of a picture is A-D converted, data obtained by the conversion is integrated by one field by an integrating circuit, digital data obtained by the integration is detected as a focus evaluating value this focus evaluating value is always compared with a focus evaluating value detected one field before, and the position of a focusing lens is controlled such that the focus evaluating value always takes the maximal value.
Such an automatic focusing apparatus is adapted such that the time change of a focus evaluating value is monitored after a focusing lens is fixed once in an in-focus position and it is determined that an object changed when the amount of the change reaches a constant amount or more, to resume an automatic focusing operation.
In a auto-focus operation by such a conventional automatic focusing apparatus, the change of the focus evaluating value obtained for each field is monitored and the position of the focusing lens is controlled such that this focus evaluating value always takes a maximum value. Thus, even if the object moves after it is brought into focus once, the focus evaluating value may not change, for example, when there is an object having a higher contrast, that is, including a higher frequency component in the background. In such a case, the auto-focus operation is not resumed, although the object actually moves so that a defocused state is achieved.
Furthermore, if and when the focus evaluating value changes after the object is brought into focus, it is difficult to determine by noticing only the change of the focus evaluating value whether the distance between the object and the lens is changed so that the focus evaluating value changes or this distance is not changed but the object laterally moves or the shape of the object itself is changed so that the focus evaluating value changes. More specifically, due to horizontal movement or vertical movement of a camera relative to objects arranged at the same distance from the lens, the change in brightness or the like, the auto-focus operation may be started by the time change of the focus evaluating value even if the distance between the object and the lens is not changed. In such a case, a picture becomes unclear.
Additionally, considering a case in which an in-focus position is detected by monitoring only the change of the focus evaluating value, when a false maximal point occurs in the focus evaluating value due to noises or the like, this maximal point is liable to be erroneously judged to be the in-focus position.
An automatic focusing apparatus for issuing a command of resuming an automatic focusing operation or a command of confirming an in-focus position by passing a level of a high-frequency component of a video signal obtained from an image sensor through a plurality of filters having different cut-off frequencies and relatively comparing levels of outputs of the filters has been proposed, which is disclosed in Japanese Utility Model Laying-Open No. 37661/1986. However, according to such a conventional technique, control is made based on the difference between absolute values of focus evaluating values and this difference is not constant depending on height of the output levels, so that a high-precision automatic focusing operation can not be carried out.
Additionally, according to the conventional technique, in order to determine whether a focus evaluating value changes after an object is brought into focus by the change in distance between the object and a focusing lens or by lateral movement of the object or the change in shape of the object itself without the change in the distance, the focusing lens is moved forward and backward by a constant amount in the direction of an optical axis to achieve a slightly defocused state when the focus evaluating value changes by a constant amount or more, and it is determined whether the distance between the object and the lens is changed depending on how the focus evaluating value changes by slight movement of the lens.
Therefore, due to horizontal movement or vertical movement of a camera relative to objects arranged at the same distance from the lens, the change in brightness or the like, the focusing lens is slightly moved to achieve a defocused state by the time change of the focus evaluating value even if the distance between the object and the lens is not changed, so that a picture becomes unclear.
Additionally, in the conventional automatic focusing apparatus, a case is considered in which an object having a large high frequency component, for example, an object having black and white stripes alternately mixed moves into a sampling area set on an image sensed picture due to horizontal movement of a video camera and movement of the object. In such a case, the focus evaluating value may be increased, although the lens moves farther away from the in-focus position. On the other hand, considering a case in which the above described object having a large high frequency component moves outside of the sampling area, the focus evaluating value may be decreased, although the focusing lens approaches the in-focus position. In such situation, the focusing lens greatly moves in an erroneous direction, so that a picture becomes unclear.